This invention relates to an iron-base alloy which, when processed in accordance with the method set forth herein, will produce an oriented grain structure in the finished product which is characterized by a cube-on-edge orientation, described in Miller Indicies as (110) [001] grain orientation, and having a primary recrystallized grain growth microstructure. Such magnetic materials are useful, for example, as core materials in power and distribution transformers.
The operating inductions of a large portion of todays transformers are limited by the saturation value of the magnetic sheet material which forms the core. In extensive use today is an iron-based alloy containing nominally 3.25% silicon (all composition percentages herein are in weight percent) which is processed in order to obtain cube-on-edge grain orientation in final product. A well-known example of this type of steel is called type M-5. These 3.25% silicon steels have the final grain orientation developed by means of a secondary recrystallized microstructure. This microstructure is obtained during the final box annealing in which preferentially oriented grains grow at the expense of non-preferentially oriented grains with the result that the alloy has an extremely large grain microstructure in which the diameter usually greatly exceeds the thickness of the sheet material. Obtaining such large grains in secondarily recrystallized microstructure requires a long time, high temperature anneal for the development of the orientation. An extensive anneal is generally also required for reduction of residual sulfur content. Sulfur contents in excess of about 100 ppm in the finished product have, in the past, adversely affected the magnetic characteristics exhibited by the silicon-iron alloy.
U.S. Pat. No. 3,833,431, issued on Sept. 3, 1974, to the inventors herein, describes a process for producing silicon steel containing nominally 3.25% silicon, by a continuous annealing process (a similar coating technique is shown in U.S. Pat. No. 3,278,348 to Foster and Seidel). The magnetic characteristics exhibited by the fully processed steel approach those of commercially available silicon steel, but without the necessity of extensively desulfurizing the steel from the sulfur content which is usually obtained by employing a commercial process. The process uses a relatively short anneal at about 1000.degree.-1100.degree. C. in order to substantially completely recrystallize the steel by a secondary recrystallization process (but which does essentially no desulfurization) and the application of a tensile stress of at least 200 psi to the steel for producing improved watt losses by, for example, applying glass to the surface of the steel sheet at an intermediate temperature (e.g. 700.degree.-850.degree. C.) such that the glass (which has a coefficient of thermal expansion substantially less than that of the steel) will, when cooled to room temperature, place the steel in tension.
The addition of the 3.25% silicon to iron, while effective and generally desirable for improving the volume resistivity, nevertheless lowers the saturation value in most commercially-produced iron alloys to generally less than about 20,300 gauss. Thus, there is a trade-off as the improved resistivity (which lowers core losses of the material) is obtained at the expense of saturation value (significantly lower than the saturation value of about 21,500 gauss of commercially pure iron). Moreover, since commercial iron has substantially higher core losses and substantially higher coercive force values than silicon steel, it was generally purdent to balance the overall magnetic characteristics and the best balance heretofore obtained was that of the 3.25% silicon iron alloy which exhibited the cube-on-edge orientation.
An alternative to the generally used commercial alloy is described in U.S. Pat. No. 3,849,212, issued Nov. 19, 1974, and the associated primary recrystallization method of U.S. Pat. No. 3,892,605, issued on July 1, 1975, (both to Thornburg) which relate to an iron-base alloy made from an ingot containing up to about 0.03% carbon, up to 1% manganese, from about 0.3 to about 4% of at least one of the volume resistivity improving elements selected from the group consisting of up to about 2% silicon, up to about 2% chromium, and up to about 3% cobalt (alloys with 4-6% cobalt are discussed in U.S. Pat. No. 3,881,967 to Cochardt and Foster), with the balance of the alloy being essentially iron with incidental impurities. Thornburg's method utilizes processing by hot working and either a two or three stage cold rolling operation with the final cold rolling stage working affecting only a moderate (50-75%) reduction in the cross-sectional area of the material being processed.